1. Field of the Invention
The present invention relates to a vinyl alcohol polymer, a method for producing the vinyl alcohol polymer, a coating agent that contains the vinyl alcohol polymer, and a coated article, such as an inkjet recording material, produced by applying the coating agent to a substrate.
2. Discussion of the Background
Vinyl alcohol-based polymers (typically referred to as vinyl alcohol polymers) (hereinafter vinyl alcohol polymer may be abbreviated as PVA) are known as water-soluble synthetic polymers, and are used as a raw material for synthetic fiber vinylon based on its strength property. In addition, they are used for paper processing agents, textile sizing agents, dispersants, adhesives, films and others, based on their excellent film-forming ability, surface activity and hydrogen bond-forming ability. Especially in the field of paper processing applications, vinyl alcohol polymers are used for improving the quality of printed matter, for example, as a surface-sizing agent for printing and writing paper; an under-sizing agent for artificial paper and coated paper; fluorescent dye dispersant; and as a filler binder for ink-jet recording materials.
Recently, inkjet printers have grown in popularity, and inkjet recording materials are used for color proofs in commercial printing, for design image outputting in the field of designing, and for sheets for over-head projectors. The demand characteristics of the inkjet recording materials for those applications are that the surface of the ink-receiving layer thereof is highly glossy, the ink-receiving layer thereof is highly transparent, the image density on these is high, their color reproducibility is good, their ink absorbability is good, and their dot reproducibility is good.
With respect to the above-mentioned characteristics for such inkjet recording materials, a recording material is known, which has, as an ink-receiving layer, a microporous layer comprising inorganic fine particles and a hydrophilic binder (JP-A 7-137434). The recording material attains both good ink absorbability and good waterproofness owing to its capillary based ink-absorbing mechanism. However, in order to form capillaries in the ink-receiving layer, in general, the layer must contain a large amount of fine particles relative to the amount of binder contained. In such a case, the ratio of binder to fine particles in the ink-receiving layer is low and therefore the layer is extremely stiff and hard. Accordingly, when a coating liquid that comprises inorganic fine particles and a hydrophilic binder is applied to a substrate to form an ink-receiving layer, internal stresses may occur when the coating layer is dried, or fine impurities may enter the layer and, as a result, the ink-receiving layer has the problem that it can be readily cracked.
To solve this problem of cracking of the coating layer, a method of thickening the coating layer before it is dried may be employed. One such method has been proposed, which comprises using a coating liquid prepared by adding boric acid as a curing agent to a polyvinyl alcohol having a high degree of polymerization, and cooling the coating layer formed from the liquid to 20° C. or lower (JP-A2001-150805). When the coating layer is cooled to 20° C. or lower in the method of JP '805, supposedly a stiff three-dimensional structure may be formed therein owing to the interaction between the polyvinyl alcohol and boric acid, and, as a result, cracking of the layer may be prevented. In this method, however, the coating layer must be cooled after it has been formed, and the energy loss in the process is great. Therefore, the method is problematic in that formation of the coating film takes a lot of time and the production speed could not be readily increased. In addition, in the method, the viscosity of the coating liquid is high since polyvinyl alcohol having a high degree of polymerization is used, and the concentration of the coating liquid must be kept low, in view of the desired handlability thereof. Accordingly, the method has another problem in that the film-drying operation takes a lot of time and the production speed could not be readily increased.
For controlling the viscosity of a coating liquid that comprises a vinyl alcohol polymer, another method of using a modified polyvinyl alcohol may be taken into consideration in addition to the above-mentioned method of adding a reactive additive to polyvinyl alcohol. There are not so many trials using a modified polyvinyl alcohol for controlling the solution viscosity behavior of polyvinyl alcohol relative to temperature change. For a polyvinyl alcohol that is responsive to heat in the form of its aqueous solution, block copolymers and graft copolymers are known, that comprise a polyvinyl alcohol component and a polyalkenyl ether component, wherein the aqueous solution shows a clouding point. A block copolymer that comprises polyvinyl alcohol and poly(2-methoxyethyl vinyl ether) has been described. The temperature dependency of the viscosity of the aqueous solution of the block copolymer within a temperature range of from 40 to 90° C. is lower than that of polyvinyl alcohol (JP-A 6-136036). A graft copolymer that comprises polyvinyl alcohol as the stem component and poly(2-methoxyethyl vinyl ether) as the branch component has also been described. The viscosity of the aqueous solution of the graft copolymer increases and the aqueous solution becomes cloudy at temperatures around the clouding point of the aqueous solution of each polymer of the stem component and the branch component of the graft copolymer (JP-A 11-322866).
One typical example of a vinyl alcohol polymer that has a polyether component and is known in the art is a saponified product of a random copolymer obtained through copolymerization of a polyoxyalkylene group-containing monomer and a vinyl ester monomer, in which the polyoxyalkylene group is essentially a polyoxyethylene or polyoxypropylene group. Based on the hydrophobic property of the polyoxypropylene group contained therein, a polyoxypropylene group-containing vinyl alcohol polymer may be used as a dispersing stabilizer which is suitable for suspension polymerization, emulsion polymerization or postemulsification (JP-A11-1505, corresponding to U.S. Pat. No. 6,107,426); as a textile sizing agent to which the amount of the oil to be added may be small (JP-A61-47878); as a composition effective for preventing paper from adhering to a blanket in offset printing (JP-A 2001-342320); and as a dispersant for heat-sensitive dyes to give thermal recording materials that have a high degree of whiteness (JP-A 4-182189). Based on the flexibility of polyoxypropylene group-containing polyvinyl alcohol, there has been developed a melt-molding method for polyvinyl alcohol containing no added plasticizer (JP-A 3-203932, corresponding U.S. Pat. No. 5,190,712); and a method of producing polyvinyl alcohol moldings that comprises contacting a polyoxypropylene group-containing polyvinyl alcohol with a boron compound, followed by melt-molding the resulting mixture (JP-A 6-256533).
However, these approaches disclose nothing about the thermal responsiveness of an aqueous solution of a polyoxypropylene group-containing vinyl alcohol polymer, and no one has heretofore made successful efforts to control the viscosity behavior, relative to temperature change, of an aqueous solution of a polyoxypropylene group-containing vinyl alcohol polymer by controlling the amount of the polyoxypropylene group in the polymer and controlling the number of the oxypropylene repeating units in the polyoxypropylene group.